1. Field of the Invention
This invention relates to electrochromic display devices utilizing a specific type of organic electrochromic material.
2. Description of the Prior Art
Electrochromic devices utilizing organic electrochromic materials have been long expected to be reduced to practice because of the advantages involved, i.e. multicolor display will be possible with clear images and the devices can be operated at low power and low voltage. However, satisfactory devices have never been in use. The main reason for this is that known electrochromic devices are not stable in operation and have not long life enough for practical applications. Typical electrochromic devices using organic electrochromic materials are described, for example, in Appl. Phys. Lett. 23 (1973), page 64, in which viologen is utilized as the electrochromic material. This device has, however, color-reversal on-off switching cycles on the order of about 10.sup.5, which are much lower than minimum practically-required cycles on the order of 10.sup.6. Presumably, this is because the deposition and redissolution phenomenon of viologen accompanied by the color formation and erasure do not proceed reversibly in a stable manner.
Another reason is that the display color is purple inherent to viologen and is not satisfactory in view of the demand for multicolor display.
Although most known electrochromic devices are monocolor displays, which show one display color on a white background, a few multicolor electrochromic devices are known. In one such device, there is used a transparent electrode on which the complex of a lanthanide metal and diphthalocyanine is vacuum-deposited in the form of a film. This device permits at least three color changes including red, green and blue but it is disadvantageous in that the device is difficult to make, the complex material is expensive, and the color change characteristics are not reliable. In addition, this device is always kept in colored state, which is disadvantageous from the practical standpoint because of the absence of the white color or colorless background which indicates a non-display condition. Other type of multicolor system is described, for example, U.S. Pat. No. 3,451,741 in which a specific type of reductant/oxidant pair is used for multicolor service. However, this system is not always satisfactory in practice.